Add plain-unit fallbacks for svmin/svh in case of unsupported engines

Couldn't reproduce the reported "hero-yes doesn't show" in a live
WebKitGTK check (svmin/svh both resolved correctly there), but an
engine that doesn't support these newer viewport units would treat
the whole declaration as invalid and drop it - for .hero-yes's height
that means falling through to auto, which collapses the section to
its in-flow content height and clips the canvas via overflow:hidden.
Declaring a plain px/vh value first means an unsupported engine keeps
that instead of falling through further.

style/main.css

@@ -54,7 +54,12 @@
      -based units recalculate as mobile Safari's address bar
      hides/shows on scroll, which would reflow every text size on the
      page mid-scroll; svmin stays fixed to the smallest viewport state,
-     same stability .hero-yes already gets from 100svh below. */
+     same stability .hero-yes gets from 100svh below.
+     Plain-px fallback declared first: an engine that doesn't
+     understand svmin/calc() here treats the whole line as invalid and
+     ignores it, leaving this in effect instead of falling through to
+     the UA default. */
+  font-size: 17px;
   font-size: max(12px, calc(2.1svmin + 3pt));
 }
 
@@ -137,6 +142,12 @@ main.loading, main.not-found {
   position: relative;
   max-width: none;
   width: 100%;
+  /* plain-vh fallback first - same reasoning as :root's font-size
+     above: an engine without svh support ignores the invalid second
+     line rather than falling through to auto height, which would
+     collapse this to the height of its in-flow content and clip the
+     canvas via overflow:hidden below. */
+  height: 100vh;
   height: 100svh;
   padding: 0;
   justify-content: flex-end;